Today, the direction of development in the area of mobile communications shows possibilities of operating cordless terminals from private branch exchanges as well. Such known wireless private branch exchanges (WPABX=Wireless Private Automatic Branch Exchanges) ("Airwaves, architecture and tomorrow's PABX", by Douglas Postlethwaite, Communications International, May 1990, p. 59-67) consist of a known private branch exchange to which wired terminals are connected as well. In its simplest configuration, the private branch exchange has one base station per radio cell, which is connected to the private branch exchange.
Comfortable configurations have a unit (RBX) performing a specified (also called "add-on") call handling function to control several base stations.
Thus, the indicated wireless private branch exchanges always consist of two independent parts, where the "mobile" part, like another subscriber as seen from the known private branch exchange, is connected to analog or digital line interfaces of the known private branch exchange.
The invention now has the task of creating a private branch exchange with an integrated mobile part.
The task is fulfilled by a private branch exchange comprising a plurality of interface modules configured for interfacing for connecting a plurality of terminals by wire and for connecting at least one base station which serves a radio cell and is connectable via an air interface to a plurality of cordless terminals, a central control unit for call handling and service-feature control in the private branch exchange, a control unit subordinate to the central control unit for processing all call-setup and -cleardown messages for a predetermined set of call numbers assigned to the cordless terminals, and a bus system to which are connected the central control unit, the subordinate control unit and the interface modules for exchanging control messages and executing switching tasks.
The availability of interface modules configured for interfacing for wired and wireless terminals, and the distribution of the control in such a way, that call handling and service feature control tasks are performed by a central control unit, and all call setup and switch-off controls for the cordless terminals are performed by a subordinate control unit, result in an integrated solution for a private branch exchange, which can control wired as well as cordless terminals with all the service features of the private branch exchange.
Since the subordinate control unit processes a specified set of call numbers that is assigned to the cordless terminals, the interface modules can be configured as ISDN-S.sub.0 interfaces, where a transfer in protocol from ISDN (Integrated Services Digital Network) to DECT (Digital European Cordless Telephone) for the cordless terminals only takes place in the assigned base station. This is important, because the ISDN- protocol can only process call numbers, so that the private branch exchange must only operate with the ISDN-protocol. This means that already existing ISDN-capable private branch exchanges can be directly used.
Thus, on the one hand, the cordless terminals can use all the service features defined in the ISDN, on the other hand however, they operate according to the DECT-protocol, so that all cordless terminals operating according to the DECT-protocol can be used, regardless of the manufacturer.
Other advantageous configurations of the subject of the invention include a private branch exchange comprising an ISDN S.sub.0 interface module (ISDN=Integrated Services Digital Network) for connecting at least one base station to make all ISDN service features available to the cordless terminals via the base station; a private branch exchange wherein the base station connected to the ISDN S.sub.0 interface module performs an ISDN-DECT protocol conversion (ISDN=Integrated Services Digital Network, DECT=Digital European Cordless Telephone), so that the cordless terminals connectable to the base station via the air interface are operable in accordance with DECT protocols; a private branch exchange wherein the subordinate control unit controls an associated terminal identification in accordance with the predetermined set of call numbers, so that only authorized cordless terminals can set up calls via the base station; a private branch exchange wherein each of a given number of call numbers of the set of call numbers is assigned to only a single cordless terminal for identifying a special authorization; a private branch exchange wherein given call numbers of the set of call numbers are assigned to two or more cordless terminals; a private branch exchange wherein the subordinate control unit contains for each base station connected via an interface module one set of call numbers each assigned to one of the cordless terminals of a radio cell, so that in response to a broadcast message from the subordinate control unit to all base stations within the ranges of the radio cells of the private branch exchange, each cordless terminal can be reached via the terminal identification (roaming); and a private branch exchange wherein when a cordless terminal is leaving one radio cell and entering another, in response to a request from the cordless terminal, the subordinate control unit can switch an existing connection with a cordless terminal to the base station associated with the other radio cell.